SJAA Ephemeris October 2001 | SJAA Home | Contents | Previous | Next

The Shallow Sky

Gas Giants and Rings

Akkana Peck


This month is a great chance to watch the gas giants of our solar system: all of them will be in the sky during normal evening viewing hours.

Throughout October, Saturn rises well before midnight and is viewable most of the night, making a nice naked-eye sight in Taurus near the Hyades cluster. Even the smallest telescope should show the rings wide open, with Cassini's division prominent; observers who wait until late in the evening when the planet is nearly overhead, where the air is steadiest, should be rewarded with good views of the translucent inner C ("Crepe") ring and the ever-elusive gaps in the outermost ("A") ring. Also look for the subtle banding on the planet's globe, and the possibility of storms which might create large light or dark spots. These come and go without warning, and I haven't heard many reports yet from observers who have seen Saturn high in the sky this year.

When you tire of looking for storms on Saturn, turn to an easier task: Jupiter, the stormiest planet in our solar system, now rising about two hours after Saturn and visible in our late evening skies. Is the Great Red Spot (the mother of all storms) continuing to redden, as appeared to be happening last year when we lost track of Jupiter? Will we be able to see as many festoons in the equatorial bands, and white swirls near the GRS, as last year? Picking up Jupiter again after a hiatus is always a treat, since the stormy planet changes so rapidly that you never know what to expect.

Well, that's not quite true. We do know we can expect to see the four Galilean moons: Io, Europa, Ganymede and Callisto, transiting across the face of their parent planet and casting shadows onto Jupiter's cloud tops. Any planetarium program can show predictions of Jovian moon events, or use my Java applet at http://www.shallowsky.com/jupiter.html, or the tables in Sky & Telescope magazine. This year, we should be able to see transits of Callisto (which is so far out from Jupiter that sometimes it appears to pass above or below the planet instead of across its face) as well as the three others.

Remember I said we could see all the gas giants this month? Well, Uranus and Neptune are both in Capricornus, and fairly easy to find. Uranus is brighter than sixth magnitude this month, which means it's visible to the naked eye from a dark site such as Fremont Peak or Henry Coe. But use binoculars or a telescope to find it if you've never looked for it before. In a telescope at moderate power, you should be able to identify it by its pale green disk - if you've ever wondered why "planetary nebulae" were given that name (they don't look anything like Jupiter!), look at Uranus and find out. Neptune is quite a bit more challenging: at magnitude 7.9 it doesn't stand out from the stars around it, and in order to see its small bluish 2.1 arcsecond face as a disk, you'll have to use fairly high power, which makes it challenging to find it in the first place (but worth the challenge).

Tired of gas giants? Okay, Mars is still visible in the early evening, racing west in Centaurus. Some observations have suggested that the near-global Martian dust storm which has obscured our view of late may be abating somewhat; observers have gotten hints of polar caps and Hellas. Alas, our tiny neighbor is also receding fast, so it will be challenging to see much detail.

Venus lays low in the morning twilight, joined by Mercury to make a nice pair in the final week of the month (before that, Mercury will be too close to the sun for easy observation). Both planets show gibbous phases to a telescope.

Oh, and don't forget to set out a telescope on the 31st to give the trick-or-treaters some "eye candy!" Unfortunately, Saturn doesn't rise until after 8pm, too late for most trick-or-treaters. But there's a full moon out (oooh, spooky!), and however jaded you may be regarding full moons, most kids (and their parents!) really get a kick out of seeing a full moon in a telescope at low power. Try filters in deep colors - dark purple, or a nebula filter - if you need to cut out some of the light to make viewing more comfortable; color filters also make the view pretty and can sometimes bring out a little more ray detail. Have fun!


Mail to: Akkana Peck
Copyright © 2001 San Jose Astronomical Association
Last updated: July 19, 2007

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